1. Field of the Invention.
The invention relates to the use of supercritical fluid processing techniques to prepare orthopedic mixtures. The mixtures can be used to make bone replacement implants or bone filler.
2. Description of the Prior Art.
There is a need for orthopedic parts such as bone implants. For example, in the to field of spinal surgery, removal of all or a portion of a damaged intervertebral disc requires that the resulting space be filled to prevent disc space collapse and to promote fusion of the adjacent vertebrae across the disc space. Desirably, the space will be filled with an implant that will have adequate strength to withstand loads imposed by the vertebrae and which will permit bone ingrowth. Although implants made of a metal such as titanium alloy have adequate strength, they have various drawbacks such as a tendency to have relatively long fusion times. Implants made of bone grafts are desirable because bone grafts are biological materials which are replaced over time with the patient""s own bone, via the process of creeping substitution. Over time a bone graft virtually disappears, unlike a metal implant which persists indefinitely.
Unfortunately, bone grafts present several disadvantages. Autogenic bone is available only in limited quantities. The additional surgery also increases the risk of infection and blood loss and may reduce structural integrity at the donor site. The graft harvesting surgery is alleged to cause extreme pain that may exceed the pain of the fusion surgery. Allogenic and xenogenic bone grafts are undesirable because they involve the implantation of a bone of foreign origin into the body, with attendant risks of infection or rejection. Although bone graft substitutes are known, as exemplified by U.S. Pat. No. 5,082,803, they have various drawbacks such as the need to cure them by the use of high temperature, high pressure steam. Desirably, a bone substitute would be available that would have the advantages of autogenic bone (such as strength and bio-compatibility) and which would be easy to manufacture and use.
In addition to the need for pre-formed bone substitutes, there is a need for high quality bone filler. In certain circumstances a bone filler can be used to repair fissures, cracks, gaps, or other defects in existing bone. Also, a bone filler can be useful in correcting certain types of bone loss that are not amenable to the use of pre-formed bone implants or for which it is difficult or impossible to create a properly shaped bone implant. Further, in the particular case of surgical procedures such as total knee or hip replacement, a bone filler is necessary to cement the implanted prosthesis into the remaining bone structure. Although bone fillers are known, as exemplified by U.S. Pat. No. 6,005,162, there remains a need for a bone filler that is easy to use, which cures quickly in place without the production of excessive heat, and which produces a strong, secure filling.
In response to the foregoing concerns, the present invention provides a technique for the manufacture of orthopedic parts such as bone implants that have the desirable characteristics of autogenic bone, but without the drawbacks of allogenic bone, xenogenic bone, or existing bone substitutes. In addition, the present invention provides for the manufacture of a bone filler having superior characteristics.
Orthopedic mixtures according to the invention are prepared by charging a reactor with starting materials that will produce an orthopedic implant or filler of desired strength characteristics. The starting materials include a source of calcium ions and a polymer or multiple polymers that forms a matrix for the calcium source. A process medium is added to the reactor. The process medium preferably is carbon dioxide which is supplied to the reactor in a supercritical state or which is heated and pressurized in the reactor to attain a supercritical state. The heated and pressurized ingredients are mixed in the reactor for a period of time sufficient to form them into a homogeneous, gas-saturated suspension, or supercritical fluid slurry. After the ingredients have been mixed adequately, the slurry either is left in the reactor or is discharged into a receiving vessel where the process medium is separated from the remainder of the materials and removed, leaving a finely divided product behind.
The finely divided product can be poured into a mold and cured by heating the mold to a relatively low temperature. Thereafter, the cured product can be used as a bone implant or a bone implant can be carved from the cured product.
In those cases where the product is to be used as a bone filler, one of the starting materials is an ultraviolet light-curable catalyst. The finely divided product can be poured or injected directly into a fissure or other gap in a patient""s bone and cured in situ by the application of ultraviolet light.
All of the orthopedic mixtures according to the invention produce a cured product that is dense, strong and porous, and which simulates autogenic bone. Typically, the cured product will have a high percentage of calcium, will be porous with 100% interconnectivity of pores (i.e., without isolated pores), and will have pore sizes on the order of 300-400 microns.